1. Field of the Invention
The invention relates to a wiring board used in an environment where high voltages from the outside such as static electricity and lightning surge are applied, as well as to a method of preventing high voltage damage of an electronic device connected to a wiring board.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2003-110212 discloses a wiring board having specified wiring patterns and mounted with an electronic device. An ICT (in-circuit tester) pad is provided for each wiring pattern near the corresponding electronic device for a test to measure characteristic values of the electronic device.
FIG. 4 is a schematic diagram of a wiring board 60 having a wiring pattern with an ICT pad. The wiring board 60 includes a wiring pattern 63 connected with an electronic device 61, such as a capacitor, shown by broken line and having two terminals. The wiring pattern 63 includes a first wiring 63a having a mounting pad 64, on which one terminal 61a of the electronic device 61 is mounted, and a second wiring 63b having a mounting pad 65, on which the other terminal 61b of the electronic device 61 is mounted. ICT wirings 67, 68 extend from the mounting pads 64, 65 and have ICT pads 69, 70 formed at their distal ends. The respective ICT pads 69, 70 are used as testing elements for characteristic measurements of the corresponding electronic device 61.
A high voltage could be applied to input and output terminals of the electronic device 61 if the wiring board 60 of FIG. 4 is exposed to static electricity or a lightning surge of several 1000 to several 10000 V and can cause an internal fracture of the electronic device 61. An internal fracture will cause the electronic device 61 to experience functional decline or functional loss.
Japanese Unexamined Utility Model Publication No. H63-80871 and Japanese Unexamined Patent Publication No. 2006-216699 disclose measures against static electricity and lightning surge. More particularly, a discharge wiring 72 shown by dashed-dotted line is connected with the first wiring 63a, a discharge wiring 73 shown by dashed-dotted line is connected with the second wiring 63b and a discharge gap 75 of a specified dimension is formed between wiring edges 72a, 73a of these discharge wirings 72, 73. A voltage V due to static electricity or lightning surge applied to the electronic device 61 is discharged from the wiring edge 72a to the wiring edge 73a via the discharge gap 75, as shown by arrows, and escapes from an unillustrated ground circuit connected with the discharge wiring 73. Thus, the electronic device 61 is protected from static electricity or lightning surge.
The discharge gap fulfills the above effect. However, it is difficult to ensure a sufficient space for a circuit pattern for forming a discharge gap since electronic devices of recent years must have higher packaging density of a wiring board.
In view of the above problem, an object of the invention is to provide a wiring board and a protection method capable of protecting an electronic device from static electricity and lightning surge without reducing the packaging density of an electronic device.